In U.S. Pat. No. 4,242,151, the applicants described a method of chromization in three successive periods set up preferably as follows:
1. An ionic nitriding at a temperature lying between 450.degree. and 650.degree. C., employing a reactive atmosphere of nitrogen and hydrogen at a partial pressure of nitrogen at most equal to 1.5 millibars and at a total gas pressure lying between 2 and 10 millibars. PA1 2. A chromization at a temperature lying between 900.degree. and 1000.degree. C. with a cement consisting of ferrochrome with a chromium content running from 50 up to 75% and the grain size of which lies between 0.5 and 4 mm, without any aluminous or magnesian binder, mixed with a halogenide. PA1 3. A heat treatment by soaking at a temperature lying between 800.degree. and 1000.degree. C. and tempering between 580.degree. and 650.degree. C. depending upon the level of resistance required for the substrate. PA1 increase of the risk of starting fissures at the interface between the two constituents M.sub.23 C.sub.6 and Cr.sub.2 (C,N), whence the possibility of scaling and localized corrosion.
It is thus possible substantially to increase the thickness of high-hardness chromized layers in the case of alloy steels containing more than 0.2% of carbon. That is, thanks to this innovation, it is possible to achieve layers of thickness equal to 50 microns whereas in the best case conventional chromidation enables one to reach 20 microns. On the other hand, the innovation in accordance with the previous patent is of equal interest to the structure of the layers:
in conventional chromization the coatings consist of two types of carbides: M.sub.23 C.sub.6, richer in chromium, towards the surface, and M.sub.7 C.sub.3, poorer in chromium, towards the metal substrate.
as a result of the treatment in three stages, which forms the object of the previous patent, the coatings consist essentially of chromium carbonitride Cr.sub.2 (C,N), of which the very high chromium content (Cr lying between 75 and 85%), the high level of hardness, lying between 2000 and 2500 Vickers (at 50 grammes) and the hexagonal structure with the base plane of the mesh directed in parallel with the surface of the substrate, form of it a compound which is particularly interesting for any problem where one is looking for high resistance to both corrosion, friction and wear.
However, under the operational conditions described in the previous patent, the obtaining of monophase layers of Cr.sub.2 (C,N) may be tricky in certain cases because of too heavy an application of chromium on the surface with respect to the flow of nitrogen proceeding from the substrate which has undergone the first phase of ionic nitriding. The result is that, along the seams of the basaltic formations of Cr.sub.2 (C,N) there is formed locally the carbide M.sub.23 C.sub.6, and the presence in the coating of this second compound has several disadvantages:
substantial increase in surface roughness;